A theoretical study of the reactivity of ethene and benzophenone with a hyper-coordinated alkene containing a so-called E=E (E = C, Si, Ge, Sn, and Pb) unit

Abstract

The reactivity of a reported hyper-coordinated alkene (Rea-E; Rea = reactant; E = group 14 element) featuring a central EE moiety was theoretically analyzed using DFT (density functional theory) and the EDA-NOCV (energy decomposition analysis–natural orbitals for chemical valence) method. M06-2X/def2-SVP and B3LYP-D3/def2-SVP results demonstrate that five Rea-E molecules have an energy minimum as their structures have no imaginary frequency. Theoretical examinations based on three types of bond order calculations (Wiberg, Mayer, and Fuzzy), the LOL (localized orbital locator) analyses, Lewis structures and the NBO (natural bond orbital) analyses suggest that a very weak central Si–Si single bond and an extremely weak central Ge–Ge single bond, rather than a double bond, are present in the Rea-Si and Rea-Ge molecules, respectively. On the other hand, no bond is found between the two central group 14 atoms in Rea-C, Rea-Sn, and Rea-Pb. The theoretical investigation demonstrates that the reactivity of the Rea-E compound decreases in the order Rea-Si > Rea-Ge > Rea-C, a trend that results from the differences in the atomic radii of the group 14 elements. Carbon has the smallest atomic radius in the group 14 family, causing steric crowding between Rea-C and other attacking species. This circumstance, in turn, increases the activation energies of its addition reactions and renders these reactions energetically infeasible. For the cyclic product of Rea-Ge, the theoretical evidence reveals that the comparatively large atomic radius of Ge induces the weakest Pauli repulsions and the smallest overlap integrals between Rea-Ge and the other doubly bonded molecules. This situation, in turn, makes the overall cyclization reaction of Rea-Ge endothermic. As a result, only the silicon-centered molecule, Rea-Si, can undergo the [2 + 2] cycloaddition reactions with doubly bonded molecules without kinetic or thermodynamic difficulty, which agrees well with the available experimental findings.